In a traditional wireless local area network, when multiple stations simultaneously send data to an access point, a collision may occur because a receiver can only receive data of a single station correctly. Carrier sense multiple access with collision avoidance (Carrier sense multiple access with collision avoidance, CSMA/CA) is generally adopted in the prior art to avoid occurrence of collision. In a new generation of wireless local area network, the problem of collision is solved in an uplink multi-user multi-input multi-output (multi input multi output, MIMO) manner. Such a receiver has capability of multi-packet reception and thus can correctly receive data sent from a plurality of stations. Furthermore, the introduction of the uplink multi-user MIMO can increase uplink throughput.
In the uplink multi-user MIMO, there are two types of access modes currently, one is completely free contention access performed by stations, the other is completely scheduling by access points to decide which station can access. However, the mode of completely free contention access by stations needs large time consumption and too complicated procedures; while the mode of completely scheduling by access points lacks flexibility. When the station has burst data to be sent, the access point cannot respond in time. Meanwhile, the two current access modes haven't considered the case that the difference between data packet lengths of the data sent by each station is too large, and there is a case that one data packet uploaded by a station is too long while another data packet uploaded by a station at the same time with the STA is too small, and this may cause a waste of the spatial stream. Moreover, in the uplink multi-user MIMO, each station does not complete data transmission at the same time, but the access point must reply an acknowledgment frame only after the data of all the stations is received. The station which completes sending will start a retransmission mechanism if the acknowledgment frame is not received within a certain period, and this will cause system errors.